A multi-axis angular rate sensor is disclosed in commonly assigned U.S. Pat. No. 4,821,572. In this device, a plurality of pairs of accelerometers are mounted on a first and a second frame member for rotation about a common axis. The first and second frame members are counterrotated about the common axis, without transmitting a reaction force to a supporting base that is interposed between the frame members.
The drive mechanism used to counterrotate the frame members, as disclosed in the above-referenced patent, comprises first and second C-shaped electromagnetic coils and associated pole pieces. Each electromagnetic coil and its associated pole piece are attached to different frame members so that when the coils are alternately and sequentially energized with an electric current, the frame members rotatably dither back and forth in opposite directions.
Several problems are associated with the drive mechanism used in the prior art multi-axis rate sensor. Although the frame members only rotate a few degrees in each direction, the first and second electromagnetic coils are energized by current supplied through leads that are continually flexed as a result of the dither motion of the device. Eventually, even the most flexible conductors available may work-harden and break. Since the coils are attached adjacent the periphery of the frame members, the mass and rotational inertia of the frame members are substantially increased by the addition of the coils, although an important design goal for this device was to minimize these parameters.
Conventional direct current (DC) electromagnetic motors capable of developing the torque required to drive the multi-axis rate sensor are comparatively bulky. Their bulk is necessary to accommodate permanent magnets, ferrous metal flux linkage members, and pole pieces that focus the magnetic flux across air gaps in the motor. A rotor in a conventional motor typically includes armature windings that are energized through brushes, which produce radio frequency (RF) noise and are subject to wear. The mass of such a rotor and its inertia prevent it from quickly stopping and reversing direction. Accordingly, a conventional prior art DC motor is not usable to drive the multi-axis rate sensor described above. Conventional motors are neither sufficiently compact nor do they include a rotor that is sufficiently low in mass and inertia to rapidly rotate back and forth through a small incremental angle. Since a conventional DC motor cannot easily be adapted to this application, it has been necessary to develop a new type of motor that meets these design criteria for driving the multi-axis rate sensor. Several embodiments of this new motor design are described in commonly assigned U.S. Pat. No. 4,968,909. The motor includes an X-shaped core and first and second pole pieces that are disposed at opposite sides of the core, i.e., above and below the core. Two opposed legs on the X-shaped core comprise a first core section, which is transverse to a similar, second core section. First and second electromagnetic coils are formed on the first and second core sections, respectively. Tabs disposed proximate the legs of the first and second pole pieces are attracted to magnetic poles created at the ends of the legs when either of the electromagnetic coils are selectively energized. Magnetic flux developed by the first and second electromagnetic coils flows through the tabs and through the first and second pole pieces, between the opposite magnetic poles. By alternately energizing the electromagnetic coils, two oppositely directed torques are sequentially developed that cause the pole pieces to counterrotate back and forth about a central axis. Since the pole pieces are lightweight, rotational inertia of the motor is very low, and its efficiency is relatively high.
Although the compact torque motor described and claimed in U.S. Pat. No. 4,968,909 solves many of the problems that preclude other motors being used to drive the multi-axis angular rate sensor, it is not an optimum solution. The configuration of the legs used in the X-shaped core was found to produce a torque having radial components. Radial components of torque are developed in this prior art configuration because the gaps defined between the generally parallel sides of the core legs and the adjacent tabs are not radially aligned in respect to the central axis of the motor. Instead, each gap is parallel with the side of one of the legs and therefore, a line through the center of the gap is offset to one side of the center of rotation or central axis of the motor. As a result, the magnetic flux through each gap develops a force that has both tangential and radial components. The radial component of force does not contribute to the desired rotation of the pole pieces and consequently represents a loss in motor efficiency. More importantly, the radial force can cause imbalance in the torque applied to the two pole pieces, producing unacceptable non-torsional stress and vibrational modes in the pole pieces that are picked up by the rotational rate sensors as noise and cause an error in the determination of rotational rate.
It is therefore an object of the present invention to minimize or compensate for any magnetic forces acting on the pole pieces in a compact motor that do not contribute to the rotational torque developed by the motor. It is a further object of the invention to provide a balanced rotational torque acting on the pole pieces of the compact motor. Yet a further object is to minimize non-torsional stress and vibrational modes that can be produced when radial forces are developed by the motor that act on the pole pieces. These and other objects and advantages of the present invention will be apparent from the attached drawings and the Description of the Preferred Embodiments that follows.